Chicago’s police superintendent on Saturday suggested that an officer’s body camera wasn’t turned on when he fatally shot a black teen last month, because the officer had only received it about a week earlier and wasn’t yet proficient in using it.

Demonstrators who held a march protesting the killing voiced strong suspicions that the camera may have been turned off as part of a cover-up.

At a news conference, Supt Eddie Johnson discussed nine videos taken from dashcams in police cars and body cameras on other officers involved in the incident. The videos show officers firing repeatedly at a stolen car as it careens down the street away from them. They also show the officers handcuffing a wounded Paul O’Neal, the unarmed 18-year-old who was driving the stolen car, after a chaotic foot chase through the city’s South Shore neighborhood.

“They had had those cameras maybe about a week … There’s going to be a learning curve,” Johnson said.

The cameras were introduced to one police district early last year as part of a pilot project. They have since been distributed to six other districts and the officer who shot O’Neal had been issued a camera as part of that rollout, said the department spokesman Anthony Guglielmi, who did not know when officers in the rest of the city’s 22 districts would be issued body cameras.

Protesters said on Saturday they did not believe any official explanation for the non-working body camera. They and the attorney representing the O’Neal family scoffed when a department spokesman said on Friday that the officer’s camera may have been deactivated by the force of the air bag when the stolen car crashed into a police cruiser.

“Since all the other cameras were working, I’m sure that camera was working and it [the shooting] was edited out or that officer turned it off on purpose,” said Ja’Mal Green, an activist who spoke to protesters at a rally. “If this is brand new equipment, how come the other officers knew to turn their cameras on and the officer who shot the fatal shot failed to turn his on or it got mysteriously turned off?”

The release of the O’Neal shooting video was the first under a new policy that calls for such material to be made public within 60 days. The policy was changed after public outrage last year following months of delay in releasing video that showed the black teenager Laquan McDonald being shot 16 times by a white officer.

The McDonald shooting video prompted accusations that Mayor Rahm Emanuel had delayed its release until after his re-election; some protesters called for him to resign. Emanuel denied he delayed the release and has refused to step down, but he fired police superintendent Garry McCarthy and replaced him with Johnson.

Johnson said the officers had training in how to use the cameras but it is not clear how extensive that training was.

“I was concerned by some of the things that I saw on the videos and that’s why we took such a swift action … that we did last week to relieve the three officers of their police powers,” Johnson said, adding he could not explain what specifically concerned him. Chicago police have not identified the officers involved.

The department’s policy prohibits officers from “firing at or into a moving vehicle when the vehicle is the only force used against the sworn member or another person”. But the policy also says that officers “will not unreasonably endanger themselves or another person to conform to the restrictions of this directive”, meaning they have the right to defend themselves if they or someone else are in imminent danger of being struck.

The department is going to look at changing training for officers and will take into account best practices from around the country, the Bureau of Professional Standards chief, Anne Kirkpatrick, said on Saturday.

The Chicago protest on Saturday coincided with the 50th anniversary of Martin Luther King’s march in the same neighborhood to protest housing segregation, during which an angry white crowd threw bottles, firecrackers and rocks, one of which struck King in the head. A memorial of the 1966 march was unveiled Friday at Marquette Park.

Bob Schwartz, a 77-year-old retired probation officer who said he marched with King in 1966, said he was discouraged about the need to march for some of the same civil rights issues that King championed.

But, he said: “It gives me encouragement to see so many young people involved in this struggle.”